1. Field of the Invention
The present invention relates to a technical field of a communication apparatus, particularly to a band-pass filter in which a film bulk acoustic resonator is used.
2. Description of the Related Art
There has been a filter using a film bulk acoustic resonator (FBAR) or thin-film piezoelectric resonator as a filter which can be used in a small size and at a high frequency. A representative equivalent circuit of the film bulk acoustic resonator is shown in FIG. 8. As seen from FIG. 8, one film bulk acoustic resonator includes series resonance portion and parallel resonance portion in the same manner as a SAW resonator. As a general method of using this resonator to constitute the filter, there is a ladder-type filter (e.g., see JP(A)-11-88111 (page 5, FIG. 18)). A frequency response of the ladder-type filter typically has a deep notch, and upper and lower pass band edges which have steep inclinations.
However, the conventional ladder-type filters tend to indicate poor stop band attenuation characteristics. That is, there is a problem that the attenuation characteristics are not sufficient in a stop band having a frequency higher or lower than that of the pass band.
In a proposed method of improving the poor stop band attenuation characteristics, an additional film bulk acoustic resonator is added to a filter constitution. However, with an increase of the number of elements, the characteristics are deteriorated in insertion loss. Moreover, with the number of resonators, the size of the whole filter increases.
JP(A) 52-19044 discloses a ladder-type filter comprising a shunt element having an equivalent parallel capacitance greater than that of a series element, which has a narrow pass band and a greater difference in lowest attenuation between the pass band and stop band. However, the elements of this filter do not use a film bulk acoustic resonator, but the filter is a SAW filter.